The goal of these investigations is to assess the relationships between developmental changes in breathing pattern and respiratory neuronal activity in the medulla and rostral pons. The animal model to be used is the opossum (Didelphis marsupialis); this marsupial mammal must utilize its lungs for gas exchange while at a generally fetal stage of development. Single unit recordings from medullary and pontine respiratory neurons will be obtained from Inactin-anesthetized animals using fluid-filled or carbon filament glass microelectrodes; breathing pattern will be determined from multiunit discharge of the phrenic nerve or diaphragm. Projection of medullary neurons to the spinal cord and vagus nerves will be evaluated after paralyzing the animals with gallamine. Specific questions to be answered include the following: 1) What is the relationship between increases in inspiratory duration as a function of maturation and the patterns as well as rates of discharge of medullary neurons? So qualitatively different types of cell discharge characterize the maturation of this system? 2) How is medullary cell discharge influenced by lung volume information from the vagus nerve? The level of airway receptor discharge is less in young animals, but certain reflexes do not appear to be compromised. 3) Do locations of respiratory neurons of the medulla change as function of maturation? Certain functional correlates of cell location are proposed for the respiratory control system. 4) Pattern of respiratory response to inhaled CO2 differs between young and adult opossums; can the mechanisms of the effect be inferred from the behavior of respiratory neurons during CO2 inhalation? 5) Rostral pontine respiratory neurons are inhibited by lung volume related afferent activity through the vagus nerves. Since such vagal activity is less in young animals, will the pontine neurons be more active and functionally more significant in the control of breathing?